1. Field of the Invention
The present invention relates in general to techniques for estimating or indirectly detecting an air pressure condition of a tire of a wheel of a motor vehicle, on the basis of a motion of the vehicle wheel, and more particularly to techniques for improving the accuracy of detection or estimation of the air pressure condition of the vehicle tire.
2. Discussion of the Related Art
There is known a technique for directly detecting the air pressure in a space within a tire of a vehicle wheel, for informing the operator or driver of a motor vehicle of the air pressure of the tire during running of the vehicle. Since the tire is rotating during the vehicle running, it is comparatively difficult to directly detect the air pressure of the tire. In view of this difficulty, a technique has been proposed for estimating the air pressure of the tire on the basis of the motion of the vehicle wheel, as disclosed in JP-A-5-133831 and U.S. Pat. No. 5,531, 110.
The technique for estimating the air pressure of the tire is based on a fact that the air pressure of the tire has a given relationship with the rigidity of the tire. The rigidity of the tire has not only a component relying on the air pressure in a space within the tire, but also a component relying on the hardness of a rubber material of the tire. These two components of the tire rigidity cannot be estimated independently of each other. It is also noted that the rubber hardness of the tire varies with the temperature of the rubber material.
However, the known technique for estimating or indirectly detecting the air pressure of the tire is not adapted to take into account a change in the tire temperature, in estimating the air pressure of the tire. In this case, the estimated air pressure of the tire differs from the actual pressure value due to a change of the rubber hardness caused by a change of the tire temperature, even if the tire air pressure estimated at a given temperature of the tire accurately represents the actual value.
The above technique for estimating the tire air pressure may be used to inform the vehicle operator that the air pressure is abnormally low. In this case, abnormally low air pressure of the tire may be estimated when the rubber hardness of the tire is excessively lowered due to a rise of the tire temperature, even if the actual air pressure of the tire is normal, that is, even where the vehicle operator should not be warned of the abnormality of the tire air pressure. Conversely, the abnormally low air pressure of the tire may not be estimated when the rubber hardness is excessively increased due to a drop of the tire temperature, even if the actual air pressure is abnormally low, that is, even where the vehicle operator should be warned of the abnormality of the tire air pressure.